It is proposed to study the structure of SV-40 and/or polyoma viruses by means of single crystal x-ray diffraction, x-ray solution scattering, and electron microscopy. SV-40 and polyoma are closely related, tumorogenic, mammalian viruses that have received intensive genetic and biochemical study. They are of particular structural interest because their genome is organized much like eukaryotic chromatin, with cellular histones being bound to viral DNA to produce a typical nucleosomal pattern. Knowledge of the structure of chromatin is crucial for the understanding of gene regulations, transcription and other cell processes. X-ray crystallography is still the most powerful tool available for the visualization of molecular structure. High resolution images of the two viruses are about to appear in print, confirming that structures weighing millions of Daltons can be successfully analyzed. Crystals of polyoma virus have been grown by Murakami, and are now under study in the laboratory of Donald Caspar, with whom I am collaborating. Crystallization of SV-40 is an urgent goal of this project. Another technique capable of yielding high resolution images is the low-dose electron microscopy pioneered by Unwin and Henderson. In this radiation damage is reduced by using periodic specimens, so that the dose that would normally pass through one element (molecule, virus, etc.) is distributed over the whole array. Two dimensional crystals of viruses are frequently seen on the electron microscope grid, but much work is needed to make these regular enough for low-dose microscopy.